Make sure there isn't anything(wires) getting in the way of the fans. Also you could unplug the case fans one at a time to isolate the one making noise. If it's a bad fan, you may be able to just get a fan replacement rather that swapping out the entire case just for a faulty case fan. I built a system abut 2 months ago with a HAF 922, and it's quiet. The only time I really hear the fans is when I first power it up.